F127-assisted hydrothermal preparation of BiOI with enhanced sunlight-driven photocatalytic activity originated from the effective separation of photo-induced carriers

Solid State Sciences 90 (2019) 1–8

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F127-assisted hydrothermal preparation of BiOI with enhanced sunlightdriven photocatalytic activity originated from the effective separation of photo-induced carriers

T

Lin Dou∗∗, Dongmei Ma, Jiufu Chen, Jianzhang Li, Junbo Zhong∗ Key Laboratory of Green Catalysis of Higher Education Institutes of Sichuan, College of Chemistry and Environment Engineering, Sichuan University of Science and Engineering, Zigong, 643000, PR China

A R T I C LE I N FO

A B S T R A C T

Keywords: BiOI Charge separation Photocatalytic activity F127 Hydrothermal method

BiOI photocatalyst with remarkably enhanced sunlight-driven photocatalytic performance was prepared by a facile hydrothermal approach with the assistance of EO106PO70EO106 (F127). Brunauer -Emmett-Teller (BET), Xray diffraction patterns (XRD), scanning electron microscopy (SEM), high-resolution transmission electron microscopy (HRTEM), Fourier-transform infrared (FT-IR), Raman, X-ray photoelectron spectroscopy (XPS), UV–Vis diffuse reflectance spectra (DRS), surface photovoltage spectroscopy (SPS) and electron spin-resonance (ESR) spectra were employed to study the samples. The level of %O2− was investigated using nitrotetrazolium blue chloride (NBT)-%O2− exclusive reaction. The results show that F127 dramatically promotes the specific area parameters, enhances the separation efficiency of photo-induced carriers and surface hydroxyl content as well as expedites the formation of %O2−. The photocatalytic activities of two photocatalysts for decomposition of methyl orange (MO) were evaluated. The photocatalytic activity of F127-BiOI is more than twice of that of BiOI.

1. Introduction Recently, sunlight-induced photocatalysis is a hot research area in the utilization of solar energy [1–8]. To effectively utilize solar energy, it is crucial to develop sunlight-response photocatalyst. Among the available sunlight-driven photocatalysts developed, bismuth oxyiodide (BiOI) has gained increasing interest because of its outstanding open layered structure and narrow bandgap [9–11]. As a p-type semiconductor, BiOI can effectively decompose organic pollutants under visible light illumination [12–14]. However, the photocatalytic activity of BiOI is far from efficient for effective large-scale applications due to its quick recombination of photo-generated charge carriers (e−-h+ pairs) and poor conductivity of photoinduced electrons [15–19]. According to the photocatalytic principle, the separation behaviors of photo-generated carriers play a leading role in determining photocatalytic efficiency [20,21]; the low separation efficiency of carriers always results in poor photocatalytic performance [22–24]. Consequently, it is indispensable to ameliorate the photocatalytic efficiency of BiOI by expediting the separation of photogenerated carriers [25,26]. To address this issue, many methods have been developed to promote

the photocatalytic performance of BiOI by facilitating the separation of photogenerated carriers, such as doping [27,28], construction of composites [29–34], crystal-facet control [35,36], morphology control [37,38] and deposition of noble metals [39,40]. All these approaches can effectively boost the photocatalytic performance of BiOI by accelerating the separation of photo-generated charge carriers. In our previous research, photocatalytic performance of BiOI can be significantly enhanced by EO20PO70EO20 (P123)-assisted hydrothermal routine. The presence of P123 in the synthetic system greatly changes the specific area parameters of BiOI, widens the bandgap and enhances the separation rate of photo-induced charge pairs, resulting in enhanced photocatalytic performance [41]. As similar compound as P123, F127 is a kind of nonionic amphiphilic block copolymer. When the amount of F127 in the synthetic system is relative small, F127 can remarkably reduce the surface tension of the solution and make the particles have a better dispersion. Moreover, as structure-directing agent, F127 can induce the formation of unique structure [42]. Consequently, it is anticipated that F127 can greatly influence the photocatalytic performance of BiOI. Inspired by above notion, herein, we report a facile F127-assisted

∗

Corresponding author. Corresponding author. E-mail addresses: [email protected] (L. Dou), [email protected] (D. Ma), [email protected] (J. Chen), [email protected] (J. Li), [email protected] (J. Zhong). ∗∗

https://doi.org/10.1016/j.solidstatesciences.2019.01.010 Received 1 November 2018; Received in revised form 25 January 2019; Accepted 30 January 2019 Available online 31 January 2019 1293-2558/ © 2019 Elsevier Masson SAS. All rights reserved.

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hydrothermal preparation to prepare BiOI with enhanced photocatalytic performance. The photocatalytic evaluation demonstrates that F127-BiOI displays enhanced photocatalytic activity under the simulated sunlight irradiation. The underlying mechanism was discussed.

Table 1 Effect of F127 on the specific surface parameters of BiOI.

2. Experimental section 2.1. Chemicals

Catalysts

SBET (m2/g)

Pore volumes (cc/g)

Pore size (nm)

BiOI F127-BiOI

4.3 17.9

0.0022 0.0072

12.2 8.0

spectrophotometer at 464 nm. To ascertain the role of active free radicals produced during the photocatalytic reaction, typical scavengers, such as isopropyl alcohol (IPA) for %OH, ammonium oxalate (AO) for h+ and benzoquinone (BQ) for %O2− were added into the MO photocatalytic systems as the procedures mentioned above, respectively.

F127 (analytical grade) was purchased from Sigma-Aldrich and used as received. All other chemicals (analytical grade) used in this work were purchased from Chengdu Kelong Chemical Reagent Factory (Chengdu, China), and used without further purification. All the studies were performed using deionized water. 2.2. Preparation of the samples

3. Results and discussion

F127-BiOI was fabricated by a typical hydrothermal method. 2 g (0.159 mmol) of F127 and 7.71 g (15.9 mmol) bismuth nitrate pentahydrate (Bi(NO3)3·5H2O) were added into 100 mL glacial acetic acid. 20 mL potassium iodide (KI) aqueous solution (2.6 g KI +20 mL H2O) was added dropwise to the above solution under vigorous stirring, forming a precipitate. The suspension system was further stirred for 20 min and then was moved into a 200 mL Teflon-lined stainless-steel autoclave. The autoclave was kept at 453 K for 24 h. The resulting product was collected by filtration, washed with deionized water and absolute ethanol, and then dispersed in absolute ethanol and dried at 353 k in air overnight. To remove the residual F127 in the sample, the powder was annealed at 673 K for 2 h, the heating rate is 2 °C/min, the corresponding sample was designed as F127-BiOI. The reference BiOI was also prepared as the same recipe mentioned above absence of F127.

3.1. Characterization of the photocatalysts The specific surface parameters of two photocatalysts were presented in Table 1. The results show that F127 definitely affects the specific surface parameters of BiOI. For BiOI, the specific surface area is 4.3 m2/g, while for F127-BiOI, the specific surface area is 17.9 m2/g. The SBET of F127-BiOI is more than four times of that of BiOI, which firmly supports that F127 inhibits the growth of BiOI, which can be further confirmed by the results of particle size distribution and XRD. It is well accepted that the specific surface area can affect the photocatalytic property of the sample. High specific surface area can offer more active sites for photocatalytic reaction, resulting in higher photocatalytic efficiency [23]. The results of specific surface parameters measurements are in good consistent with the results of the photocatalytic evaluation. Adding F127 into the synthetic system of BiOI significantly influences the particle size distribution, as shown in Fig. 1, D50 for BiOI and F127-BiOI is 25.16 μm and 7.05 μm, respectively. Usually, low D50 value corresponds to high BET specific surface area [45], which accords well with the results of BET surface area. Fig. 2A displays the XRD profiles of BiOI and F127-BiOI. It is obvious that the diffraction peaks of two samples can be indexed to the tetragonal BiOI, which coincides well with the standard JCPDS file of BiOI (No.10-0445) [46]. No additional diffraction peaks of impurity, such as Bi2O3 or Bi(OH)3, were detected, indicating high purity of the as-prepared samples. Moreover, it is interesting to note that full width at half maximum (FWHM) of (001), (102) and (004) plane of F127-BiOI is larger than that of BiOI, which further certifies that F127 restrains the growth of BiOI. Fig. 2B clearly illustrates the change of some peaks. According to Scherrer formula: D = Kλ/Bcosθ, where D is crystalline size, K is a constant (0.9), λ is 1.5406 Å, B is the FWHM measured in radians on the 2θ scale, and θ is the Bragg angle for the diffraction peaks. Wider FWHM leads to lower crystal size [47]. The FWHM and crystal size of BiOI crystal were presented in Table 2, the average crystal sizes of BiOI and F127-BiOI are estimated to be 24.90 nm and 23.16 nm, respectively. Relative small crystal size results in high BET surface area [48], fitting well with the result of BET surface area. As shown in Fig. 3A and Fig. 3B, both BiOI and F127-BiOI display plate-like shape, however, compared to BiOI, F127-BiOI holds smaller particle size, resulting in high SBET. As nonionic amphiphilic block copolymer, F127 can remarkably reduce the surface tension of the solution, thus BiOI particles have a better dispersion, manifesting relative small particle size, proven by BET, XRD and particle size distribution. TEM and HRTEM images were displayed in Fig. 3C and D. As shown as in Fig. 3C, plate-like structure was observed, which can be assigned to the layered structure of F127-BiOI. The interplanar space of 0.2413 nm is in good consistent with the (113) plane of F127-BiOI. The result of EDS (Fig. 3E) shows that O, Bi, and I were detected on the surface of F127-BiOI sample, these elements originate from BiOI. All these results provide strong evidence for the successful preparation of BiOI.

2.3. Characterization of the samples XRD patterns of the samples were characterized on a DX-2600 X-ray diffractometer at 35 kV and 25 mA with Cu Kα (λ = 0.15406 nm) radiation. The specific surface area and pore size were measured on an SSA-4200 automatic surface analyzer (Builder, China). The particle size distribution was recorded on a BT-9300H laser particle size distribution analyzer. The morphologies of the samples were observed on a JSM7500F scanning electron microscope. TEM and HRTEM images were obtained on a Tecnai TEM G2 microscope at 300 kV. UV–Vis DRS of the samples were carried out on a TU-1950 spectrophotometer equipped with an integrating sphere. The measurements of SPS were carried out following the recipes described in Ref. [43]. Raman spectra were measured on a laser micro-Raman spectrometer (Thermo DXR Microscope, USA) using 780 nm laser as excitation. X-ray photoelectron spectroscopy (XPS) spectra were acquired on an ESCALAB MKII X-ray photoelectron spectrometer. ESR spectra were performed on a Bruker E 500 spectrometer using 5,5-dimethyl -1- pyrroline-N-oxide (DMPO) as capture. The level of %O2− was investigated using NBT method as described in Ref. [44]. 2.4. Photocatalytic reaction Photocatalytic elimination of MO was performed in a Phchem III photochemical reactor (Beijing NBET Technology Co., Ltd, China) under intense stirring. The irradiation source was a 500 W Xe lamp (simulated solar light). Quartz test tubes were located around the lamp, and the distance from the lamp to the quartz test tubes was about 10 cm. The photocatalytic reaction was performed at room temperature. The dosage of photocatalyst was 1 g/L, the initial mass concentration of MO aqueous solution was 10 mg/L, the volume of MO aqueous solution was 50 mL, the initial pH value of MO aqueous solution was 7.0. At regular timespans, the suspension system was sampled for analysis. The concentration of MO was determined on a TU-1950 2

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(A)

Intensity (a.u.)

F127-BiOI

BiOI PDF#10-0445 10

20

30

40

50

60

(004)

(B)

(013)

(110)

Intensity (a.u.)

(102)

2 Theta (degree)

F127-BiOI BiOI 28

30

32

34

36

38

40

2 Theta (degree) Fig. 2. XRD patterns of the samples (A); an enlarged view of the diffraction region between 28°and 41° (B). Table 2 The FWHM and crystal size of BiOI crystal.

Fig. 1. Distribution of particle size; (A) BiOI; (B) F127-BiOI.

Facet

The surface chemical composition of BiOI samples were investigated by XPS. The spectra were illustrated in Fig. 4. It can be seen from the XPS survey spectra (Fig. 4A) that the as-prepared samples contain C, Bi, O and I elements. C 1s (292.2 eV) can be ascribed to the adventitious carbon from the XPS instrument. As exhibited in Fig. 4B, the peaks located at 159.12 eV and 164.45 eV belong to the binding energies of Bi 4f7/2 and Bi 4f5/2, respectively. Two peaks at 630.7 eV and 619.2 eV (Fig. 4C) can be assigned to I 3d3/2 and I 3d5/2 [49–51], respectively. The high-resolution XPS spectra for O 1s region were demonstrated in Fig. 4D and E, two peaks situated at 530.21 eV and 533.10 eV can be assigned to Bi-O in (BiO)22+ slabs of BiOI layered structure and hydroxyl groups on the surface [52], respectively. Surface hydroxyl group exists in BiOI is attributed the chemically adsorbed H2O. Furthermore, the content of surface hydroxyl oxygen on F127-BiOI (33.94%) was higher than that on BiOI (11.74%), implying that the existence of F127 in the synthetic system can significantly enhance the surface hydroxyl content. Generally, high content of surface hydroxyl on the surface of BiOI is conducive to the enhancement of photocatalytic activity [16], which can be further supported by the results of photocatalytic evaluation. To further confirm the successful preparation of BiOI, Raman

2 Theta/degree FWHM crystal size/nm

(001)

(102)

(004)

F127-BiOI

BiOI

F127-BiOI

BiOI

F127-BiOI

BiOI

9.658 0.337 23.36

9.658 0.318 24.75

29.645 0.388 23.01

29.645 0.358 24.93

39.365 0.433 23.11

39.365 0.401 25.03

spectra of the as-prepared samples were shown in Fig. 5. Two samples show Raman band at 95.9 cm−1 and 159.3 cm−1, respectively, which can be assigned to Eg of Bi-I stretching mode [53]. No other peaks are observed, implying no other functional groups are formed in F127-BiOI. The result substantially proves that F127 was removed completely from F127-BiOI sample. The FT-IR spectra of BiOI and F127-BiOI samples were shown in Fig. 6. The peak at 1629 cm−1 can be assigned to the δ (eOH) bending vibration of the surface hydroxyl group on the photocatalyst surface [54]. Hydroxyl groups existed on the surface of two photocatalysts stems from chemically adsorbed H2O. As shown in Fig. 6, the δ (eOH) bending vibration of the surface hydroxyl group on F127-BiOI is stronger than that on the surface of BiOI, which indicates the surface hydroxyl content on F127-BiOI is higher than that on BiOI. The results 3

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A

B

D

C

1000

(E)

Bi

Intensity (a.u.)

800 600 400

I

200 O 0

Bi 1

I

Bi

2 3 Energy (keV)

I

4

5

Fig. 3. SEM of BiOI (A); SEM of F127-BiOI (B); TEM of F127-BiOI (C); HRTEM of F127-BiOI (D); EDS of F127-BiOI (E).

which demonstrates that the photogenerated carriers can be effectively separated under the irradiation of visible light. The results in this case indicate that F127 alters the surface states of BiOI, expediting the separation of photoinduced carriers. High separation rate of photoinduced carriers is advantageous to produce more active species during the photocatalytic process [59], resulting in higher photocatalytic performance. To detect the active free radicals formed during the photocatalytic process, ESR experiments were performed. As shown in Fig. 9, ·O2− radicals were observed for the F127-BiOI photocatalytic system, while no signals of ·OH radicals were observed, indicating that no ·OH radicals were formed. The ESR results are consistent with the trapping experiments. To further ascertain the role of active free radicals during the photocatalytic process, trapping experiments were performed. As shown in Fig. 10 (inset), scavengers execute different effects on the decay of MO over F127-BiOI. Adding BQ into the photocatalytic reaction system significantly inhibit the decay of MO, suggesting that ·O2− is the leading active free radicals. Superoxide radicals (·O2−) has a strong oxidative ability and take a crucial role in the photodegradation reactions, the formation of ·O2− facilitates the charge separation in the photocatalytic process [60–63]. The results further display that holes take a secondary role. To further compare the level of ·O2− in the different photocatalytic systems, NBT measurements were carried out. As

are consistent with the results of XPS. Generally, the enhancement of surface hydroxyl content on the photocatalyst is favorable for the photocatalytic activity [55]. UV–Vis DRS of the samples were demonstrated in Fig. 7A. It can be observed that both BiOI and F127-BiOI have excellent absorption in the visible light region. The indirect bandgaps of two samples were estimated by fitting a plot of (αhv)1/2 versus hv, where α is absorbance. Fig. 7B shows the indirect bandgap for F127-BiOI and BiOI is 1.85 eV. The results firmly show that the bandgap cannot be altered by F127. The band positions of F127-BiOI can be further calculated following the formulas [56]: EVB = X - Ee + 0.5 Eg, ECB = EVB - Eg, where EVB and ECB represent the valence band (VB) and conduction band (CB) edge potential, respectively. X is the electronegativity of the semiconductor; Ee is the energy of free electrons on the hydrogen scale (∼4.5 eV). X is 5.94 eV for BiOI [57]. Based on the above equation, the VB and CB edge potential of F127-BiOI are 2.37 eV and 0.52 eV vs NHE, respectively. The SPS responses of two photocatalysts were exhibited in Fig. 8. It can be seen that BiOI and F127-BiOI samples display apparent SPS response from 300 to 650 nm, which is attributed to the electrons jump from the VB to the CB under simulated sunlight irradiation. Moreover, F127-BiOI sample has stronger SPS response than BiOI, indicating that the separation rate of photogenerated carriers is higher than BiOI based on the working principle of SPS [58]. Furthermore, enhanced SPS response in the region 420-650 nm was observed for F127-BiOI sample,

4

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200

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10000 BiOI

0

0

168

(C)

F127-BiOI

20000

11000

10000

Intensity (a.u.)

11000

636 632 628 624 620 Binding energy (eV)

O 1s BiOI

3600 3000 2400 1800 1200 600 -1 Wavenumber (cm )

-OH

536

F127-BiOI

20

Bi-O

9000

616

40

(D)

10000

BiOI 0

164 160 156 Binding energy (eV)

534

532 530 528 Binding energy (eV)

526

Fig. 6. FT-IR spectra of the samples.

(E)

O 1s F127-BiOI

10000

Bi-O

9000 536

Absorbance (a.u.)

Intensity (a.u.)

I 3d5/2

I 3d3/2

Intensity (a.u.)

I 3d

BiOI

F127-BiOI

20000

Binding energy (eV) 30000

80

(B)

496

800

Bi 4f7/2

Bi 4f5/2

T (%)

1000

Intensity (a.u.)

Bi 4f

I 4d

BiOI 0

Bi 5d

F127-BiOI

100000

C 1s

Bi 4p I 3d O1s Bi 4d

200000

Bi 4f

1629.15

30000

(A) I(MNN) I 3p

Intensity (a.u.)

300000

-OH 534

532

530

528

526

Binding energy (eV)

Fig. 4. XPS spectra of the photocatalysts: survey XPS spectrum of BiOI (A); Bi 4f (B); I 3d (C); High-resolution XPS spectra of O 1s BiOI (D) and (E) F127-BiOI.

600

Intensity (cps)

500 400 300

(A)

(A)

BiOI

0.6

F127-BiOI

0.4

0.2

95.9 159.3 (B)

F127-BiOI BiOI

300

400

500 600 700 Wavelength (nm)

800

200 4

100 -100

1/2 1/2 ( hv) /(eV)

0 800 1600-1 100 200 -1 Raman shift (cm ) Raman shift (cm )

Fig. 5. Raman spectra of the samples (A); the partial enlargement (B).

displayed in Fig. 10, the concentration of NBT in the F127-BiOI photocatalytic system is lower than that in the BiOI photocatalytic system due to the exclusive reaction between NBT and ·O2−, which firmly suggests that F127-BiOI photocatalytic system yields more ·O2− than BiOI photocatalytic system. The photogenerated electrons of BiOI can be excited up to a higher potential edge (−0.56 eV) [64,65]. The redox potentials for O2/·O2− is −0.33 eV, thus the photogenerated electrons can react with O2 to yield ·O2−. In view of ·O2− plays a predominant role in the decay of MO, high level of ·O2− will expedite the removal of MO, displaying high photocatalytic activity.

(B) BiOI

3 2

F127-BiOI

1.85 eV 1 0 1.6

1.8

2.0

2.2

2.4

2.6

2.8

3.0

Energy (eV) Fig. 7. UV–Vis DRS of the samples (A); Bandgap of the samples (B).

3.2. Photocatalysis The photocatalytic activities of BiOI and F127-BiOI was tested and presented in Fig. 11. As shown in Fig. 11, even considering the 5

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1.0

F127-BiOI

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Light on

BiOI F127-BiOI

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Photovlotage (uV)

L. Dou et al.

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BiOI

50 0 300

350

0.6 0.4

400 450 500 550 Wavelength (nm)

0.2

600

-1

Fig. 8. SPS response of the photocatalysts.

0

1 2 3 4 Irradiation time (h)

5

Fig. 11. MO degradation activities.

adsorption of MO over the samples, the removal of MO over different photocatalysts under the simulated sunlight illumination stems from the photocatalysis. F127-BiOI exhibits higher photocatalytic activity than BiOI. The decay of MO on two catalysts accords well with firstorder kinetics, the apparent removal rate constants for MO over BiOI and F127-BiOI are 0.108 h−1 and 0.229 h−1, respectively. Clearly, the photocatalytic activity of F127-BiOI is more than twice of that of BiOI.

Intensity (a.u.)

-

DMPO- O2

DMPO- OH

3300

3.3. Photocatalytic mechanism

3320 3340 3360 3380 Magnetic field (G)

Based on all the information, the band structure diagram of F127BiOI was presented in Fig. 12. F127-BiOI can be excited to form electron-hole pairs under visible light (λ＞420 nm, energy less than 2.95 eV). The CB bottom and VB top of F127-BiOI are about 0.52 eV and 2.37 eV vs. NHE, respectively. The reduction potential of O2/·O2− is −0.33 eV, it is apparent that the electrons from CB of F127-BiOI cannot reduce O2 to form ·O2−. However, electrons can be excited up to a higher potential edge (−0.56 eV) under visible light illumination [65,66]. The reformed CB edge potential of F127-BiOI (−0.56 eV) is more negative than that of the reduction potential of O2/·O2−, Consequently, the photogenerated electrons from the reformed CB of F127BiOI can easily react with O2 adsorbed on the surface of F127-BiOI to produce ·O2−. The formation of ·O2− can effectively inhibit the recombination of electron-hole pairs, thereby the holes can also move to

3400

Fig. 9. DMPO spin-trapping ESR spectra recorded with F127-BiOI sample in methanol dispersion (for DMPO-%O2−) and in water dispersion (for DMPO-% OH)) under simulated sunlight illumination for 2min. 40

32.0 Decomposion (%)

Absorbance (a.u.)

0.3

Blank

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BiOI

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20

12.6 10 0

Blank

IPA

AO

BQ

Scavenger

0.1 0.0

30

F127-BiOI 250

300

350

400

Wavelength (nm) Fig. 10. Absorbance of NBT in the different photocatalytic system (illumination time = 30 min, NBT dosage = 0.05 mmol/L), the inset is the effects of scavengers on the photocatalytic decolorization efficiency of MO over F127-BiOI (illumination time = 60 min, scavenger dosage = 0.2 mmol/L). Fig. 12. Photocatalytic mechanism of F127-BiOI. 6

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surface of BiOI [57], inducing the photocatalytic reaction. The VB potential of F127-BiOI (2.37 eV) is more negative than the standard reduction potential of ·OH/OH− (2.68 eV), suggesting that the h+ on the surface of F127-BiOI cannot oxidize OH− into ·OH. Therefore, no ·OH radicals were formed, which fits well with ESR and trapping experiments. ·O2− and h+ formed can result in the degradation of MO. In summary, in light of all the results in this paper, the enhanced photocatalytic activity of F127-BiOI can be assigned to the high specific surface area, high surface hydroxyl content and high separation rate of photo-induced charge carriers and high level of ·O2−.

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